I just up graded from 0.2.18 to 0.2.26 and it went fine. It said to upgrade DB and I did. Everything looked good until I clicked on the "downloads" widget and it would not drop down to display data. It would drop down 1/8 inch and kind of shake the page in the browser (Firefox). The same thing with "List of External Websites", and "Best search engines". Also on the bottom of the widgets the dialogs stating such as "1-5 out of 5", "1-5 out of 10", etc. were gone. Moving a widget to the admin area and testing did not help. Deleted 0.2.26 and reinstalled 0.2.18 and everything is working. Sorry I can not help much more as I am code challenged, but can answer questions.
I just did what I thought you would ask me to do and deleted all the files from the old install (except config.ini)before uploading the new install. Same thing except I described it wrong. When you click on the widgets noted above, the drop down stays stuck at "loading". Went back to 0.2.18 and all works again.
Save a copy of your config.ini.php (from your current 0.2.18 installation).
Upgrade to 0.2.28. Save another copy of your config.ini.php (post-upgrade).
If the GUI problem persists, delete config/config.ini.php, open your browser to your piwik (where it'll step you thru a re-install), and select the option to keep your tables. Save this new version of config.ini.php. (post-reinstall)
Remove the sensitive information from your backups of config.ini.php and then attach them to this ticket. Thanks.
I feel stupid. I upgraded 2.18 to 2.28 and some widgets worked, some displayed your Opps thingy, and some stuck on loading... I "shift - refreshed" the page and the widgets worked, but some data was missing from earlier in the day. The back up install I created from the old install in a newly created folder, worked. I then figured it out. The new install defaulted to "yesterday" and the old install to "today". The poor browser did not know what to think!
One more ticket closed that should not have been there.
Closing per requester.